Golf ball

ABSTRACT

In a golf ball having a core of one or more layer, an intermediate layer and a cover, the cover consists of an inner cover layer and an outer cover layer, the inner cover layer and the outer cover layer being made of materials having hardnesses on the Shore D hardness scale of, respectively, from 20 to 40 and from 35 to 55, and the inner and outer cover layers having a combined thickness of from 0.6 to 1.2 mm. Also, letting (A) and (B) be the rebound resiliences (%) of, respectively, the inner cover layer material and the outer cover layer material based on JIS-K 6255, the ball satisfies the condition 0%&lt;(A)−(B)&lt;18%. This ball exhibits a higher spin rate on approach shots and a good feel without lowering the distance performance on shots with a driver (W #1).

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2022-062754 filed in Japan on Apr. 5, 2022, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a golf ball having a multilayer construction that includes a core of one layer or a plurality of layers, an intermediate layer, and a cover composed of two layers—an inner cover layer and an outer cover layer.

BACKGROUND ART

The property most desired in a golf ball is an increase in the distance traveled by the ball, although another desired property is the ability for the ball to stop well on approach shots. In order for a golf ball to have such properties, it has been common of late to use a urethane resin material as the cover material in the ball. Thermoplastic urethane elastomers in particular are often used as the urethane resin material, and their properties are constantly being upgraded. A number of disclosures have been made that further improve the properties of cover-forming resin compositions by using a thermoplastic urethane elastomer as the base resin while including also other resins and additives.

For example, to improve the scuff resistance of the cover material, JP-A H11-9721 discloses the use of a blend composed of a thermoplastic polyurethane and a styrene-based block copolymer as the base resin of the cover material. However, covers made of this blend are inadequate in terms of their rebound resilience and scuff resistance. In another disclosure, JP-A 2017-12737 describes a golf ball in which a urethane-based resin composition is softened by including therein a plasticizer. However, the resin composition in this case merely becomes softer and a sufficient increase in the spin rate on approach shots is not achieved.

Additionally, when blending a urethane resin material with another resin material for the purpose of lowering the hardness, it is desirable to avoid to the extent possible any changes in the rebound resilience and a worsening of the moldability.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a golf ball which exhibits a higher spin rate on approach shots and a good feel without lowering the distance performance on shots with a driver (W #1).

As a result of intensive investigations, we have discovered that in order to improve the controllability on approach shots of a golf ball having a core, an intermediate layer and a cover (outermost layer), rather than taking an approach commonly used up until now such as blending various other polymers and additives in a urethane resin material, certain desirable effects can be achieved by instead forming a relatively thin layer in the manner of a film inward of the golf ball cover (outermost layer) and either selecting for this thin layer a material that has a rebound resilience and a hardness within specific ranges or specifying the rebound resilience relationship between the thin layer and the cover (outermost layer). Namely, a golf ball having both this outermost layer and this thin layer is able to achieve a high spin rate on approach shots and also has a good feel on approach shots. Moreover, when the ball is struck with a driver (W #1), there is no loss of distance performance, making it an advantageous golf ball for golfers in general.

Accordingly, the invention provides a golf ball having a core of one or more layer, an intermediate layer and a cover, wherein the cover consists of an inner cover layer and an outer cover layer, the inner cover layer being made of a material having a hardness on the Shore D hardness scale of from 20 to 40, the outer cover layer being made of a material having a hardness on the Shore D hardness scale of from 35 to 55 and the inner and outer cover layers having a combined thickness of from 0.6 to 1.2 mm, and wherein, letting (A) be the rebound resilience (%) of the inner cover layer material based on JIS-K 6255 and (B) be the rebound resilience (%) of the outer cover layer material based on JIS-K 6255, the ball satisfies the condition 0%<(A)−(B)<18%.

In a preferred embodiment of the golf ball of the invention, the outer cover layer material has a resin component which is a thermoplastic polyurethane elastomer.

In another preferred embodiment of the inventive golf ball, the inner cover layer material has a resin component which is a thermoplastic elastomer.

In yet another preferred embodiment, the inner cover layer material has a resin component which is a multi-component copolymer having conjugated diene units, non-conjugated olefin units and aromatic vinyl units, the conjugated diene units including butadiene units, the non-conjugated olefin units including units selected from ethylene units, propylene units and 1-butene units, the aromatic vinyl units including styrene units, and the content of the conjugated diene units in the multi-component copolymer being at least 5 wt %. In this preferred embodiment, the content of the conjugated diene units in the multi-component copolymer may be at least 10 wt %, the content of the non-conjugated olefin units in the multi-component copolymer may be up to 90 wt %, the content of the aromatic vinyl units in the multi-component copolymer may be up to 30 wt %, the non-conjugated olefin units may be ethylene units, and the multi-component copolymer may be a copolymer polymerized with a gadolinium metallocene complex catalyst.

In still another preferred embodiment, the inner cover layer has a thickness of from 0.1 to 0.5 mm.

In a further preferred embodiment, the intermediate layer, the inner cover layer and the outer cover layer have a thickness relationship therebetween which satisfies the following condition:

intermediate layer thickness>inner cover layer thickness<outer cover layer thickness.

In a still further preferred embodiment, the ratio (a)/(b) between the inner cover layer thickness (a) and the outer cover layer thickness (b) is from 0.10 to 0.60.

ADVANTAGEOUS EFFECTS OF THE INVENTION

The golf ball of the invention exhibits a higher spin rate on approach shots and a good feel without lowering the distance performance on shots with a driver (W #1).

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic cross-sectional view of a golf ball according to one embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the appended diagram.

The golf ball of the invention is, as shown in FIG. 1 , a golf ball G which has a core 1, an intermediate layer 2 encasing the core 1, an outer cover layer 4 encasing the intermediate layer 2, and a thin inner cover layer 3 inward of the outer cover layer 4. The core 1 and the intermediate layer 2 can each be formed as a single layer or a plurality of layers. Numerous dimples D and a coating layer 5 are normally formed on the surface of the outer cover layer 4.

The core may be formed using a known rubber material as the base material. A known base rubber such as natural rubber or synthetic rubber may be used as the base rubber. More specifically, it is recommended that polybutadiene, especially cis-1,4-polybutadiene having a cis structure content of at least 40%, be chiefly used. If desired, natural rubber, polyisoprene rubber, styrene-butadiene rubber or the like may be used together with the foregoing polybutadiene in the base rubber.

The polybutadiene may be synthesized with a metal catalyst, such as a neodymium or other rare-earth catalyst, a cobalt catalyst or a nickel catalyst.

A co-crosslinking agent such as an unsaturated carboxylic acid or a metal salt thereof, an inorganic filler such as zinc oxide, barium sulfate or calcium carbonate, and an organic peroxide such as dicumyl peroxide or 1,1-bis(t-butylperoxy)cyclohexane may be included in the base rubber. If necessary, a commercial antioxidant or the like may be suitably added.

The core has a diameter which, although not particularly limited, is preferably at least 20 mm, more preferably at least 25 mm, and even more preferably at least 30 mm. The diameter is preferably not more than 41 mm, and more preferably not more than 40 mm.

The intermediate layer has a material hardness on the Shore D hardness scale which, although not particularly limited, may be set to preferably at least 55, more preferably at least 60, and even more preferably at least 65. The material hardness may be set to preferably not more than 70.

The intermediate layer has a thickness which, although not particularly limited, is preferably at least 0.5 mm, more preferably at least 0.6 mm, and even more preferably at least 0.7 mm. The thickness is preferably not more than 1.8 mm, more preferably not more than 1.7 mm, and even more preferably not more than 1.6 mm.

Although not particularly limited, a known thermoplastic resin or thermoplastic elastomer may be used as the intermediate layer material, with the use of an ionomer material being especially preferred. Preferred use can be made of a commercial ionomer resin or of a resin mixture which includes:

100 parts by weight of a resin component consisting of, in admixture,

-   -   (i) a base resin of         -   (i-1) an olefin-unsaturated carboxylic acid random copolymer             and/or a metal ion neutralization product of an             olefin-unsaturated carboxylic acid random copolymer blended             with         -   (i-2) an olefin-unsaturated carboxylic acid-unsaturated             carboxylic acid ester random terpolymer and/or a metal ion             neutralization product of an olefin-unsaturated carboxylic             acid-unsaturated carboxylic acid ester random terpolymer         -   in a weight ratio (i-1):(i-2) of between 100:0 and 0:100,             and     -   (ii) a non-ionomeric thermoplastic elastomer     -   in a weight ratio (i):(ii) of between 100:0 and 50:50;         and which also includes, blended therewith:     -   (iii) from 5 to 80 parts by weight of a fatty acid and/or a         fatty acid derivative having a molecular weight of from 228 to         1,500, and     -   (iv) from 0.1 to 17 parts by weight of a basic inorganic metal         compound capable of neutralizing un-neutralized acid groups in         components (i) and (iii).         In particular, when using a mixture of above ingredients (i) to         (iv), it is preferable to use one in which at least 70% of the         acid groups are neutralized.

In this invention, an inner cover layer which inwardly adjoins the subsequently described outer cover layer is formed. This layer is thin in the manner of a film and has a suitable rebound resilience.

The inner cover layer has a thickness which is preferably at least 0.1 mm, and more preferably at least 0.2 mm. The thickness is preferably not more than 0.5 mm, and more preferably not more than 0.4 mm. At a thickness lower than the above range, a high spin rate on approach shots with a club such as a sand wedge (SW) may not be obtained. On the other hand, at a thickness greater than the above range, the spin rate on shots with a driver (W #1) may rise, as a result of which an increased distance may not be achieved, or the feel of the ball may worsen.

The inner cover layer has a material hardness, in terms of the Shore D hardness scale, which is 20 or more, preferably 24 or more, and more preferably 28 or more. The upper limit is not more than 40, preferably not more than 38, and more preferably not more than 36. When the material hardness of this inner cover layer is lower than the above range, the spin rate on shots with a driver (W #1) rises and so an increased distance cannot be achieved. On the other hand, when the material hardness of this inner cover layer is higher than the above range, a high spin rate cannot be obtained on approach shots with a club such as a sand wedge (SW) and the feel of the ball worsens.

The inner cover layer material has a rebound resilience, as measured in accordance with JIS-K 6255, which is preferably at least 50%, and more preferably at least 55%. The upper limit is preferably not more than 75%, and more preferably not more than 70%. When the rebound resilience is too low, the spin rate on approach shots rises, as a result of which the distance traveled by the ball on shots with a driver may decrease. On the other hand, when the rebound resilience is too high, the high spin rate desired on approach shots may not be attainable. The rebound resilience of the inner cover layer material is always set higher than the rebound resilience of the subsequently described outer over layer.

The inner cover layer material may use as the resin component therein any of various types of thermoplastic elastomers, or may use a multi-component polymer having conjugated diene units, non-conjugated olefin units and aromatic vinyl units. This multi-component copolymer is the multi-component copolymer disclosed in JP No. 6780827, and has in the present invention an optimal rebound resilience and other properties. The multi-component copolymer is described below.

Conjugated Diene Units

The multi-component copolymer includes conjugated diene units. The conjugated diene units are structural units from a conjugated diene compound serving as a monomer. Because the multi-component copolymer can be polymerized using a conjugated diene compound as a monomer, compared with copolymers obtained by polymerization using a known non-conjugated diene compound such as EPDM, it has excellent crosslinking properties. The conjugated diene compound includes a butadiene unit. The butadiene unit is a structural unit from a butadiene compound. Specific examples of the butadiene compound include 1,3-butadiene, isoprene, 1,3-pentadiene and 2,3-dimethyl-1,3-butadiene. The conjugated diene units in the multi-component copolymer preferably include 1,3-butadiene units, and more preferably consist solely of 1,3-butadiene units.

The multi-component copolymer has a cis-1,4 bond content in the overall conjugated diene units that is preferably at least 50%, more preferably at least 70%, even more preferably at least 80%, and still more preferably at least 90%. Such a multi-component copolymer having a high cis-1,4 bond content in the overall conjugated diene units can be obtained by using as the monomers a conjugated diene compound, a non-conjugated olefin compound and an aromatic vinyl compound. The content of vinyl bonds (1,2-vinyl bonds, 3,4-vinyl bonds, etc.) in the conjugated diene units overall is preferably 30% or less, more preferably 15% or less, even more preferably 10% or less, and still more preferably 6% or less. Also, the trans-1,4 bond content in the overall conjugated diene units is preferably 30% or less, more preferably 15% or less, and even more preferably 10% or less. The respective contents of cis-1,4 bonds, trans-1,4 bonds and vinyl bonds can be determined by the integrated area ratios from ¹H-NMR and ¹³C-NMR measurement results.

The conjugated diene compound may be of one type used alone or two or more types may be used together. That is, the multi-component copolymer may include one type of conjugated diene unit alone or may include two or more types. The content of conjugated diene units is preferably at least 10 wt %, and more preferably at least 15 wt %, of the overall multi-component copolymer. The content of conjugated diene units is preferably 80 wt % or less, more preferably 60 wt % or less, and even more preferably 50 wt % or less, of the overall multi-component copolymer.

Non-Conjugated Olefin Units

The multi-component copolymer includes non-conjugated olefin units. The non-conjugated olefin units are structural units from a non-conjugated olefin compound serving as a monomer. The non-conjugated olefin compound is selected from the group consisting of ethylene, propylene and 1-butene. In particular, to fully impart the golf ball material with rebound resilience and softness, it is preferable for the non-conjugated olefin units to be ethylene units.

The non-conjugated olefin compound may be of one type used alone, or two or more types may be used together. That is, the multi-component copolymer may contain one type of non-conjugated olefin unit, or may contain two or more types. The content of non-conjugated olefin units is preferably more than 20 wt % and less than 90 wt %, more preferably from 30 to 85 wt %, even more preferably from 40 to 80 wt %, and most preferably from 45 to 75 wt %, of the overall multi-component copolymer.

Aromatic Vinyl Units

The multi-component copolymer includes aromatic vinyl units. The aromatic vinyl units are structural units from an aromatic vinyl compound serving as a monomer. Specific examples of the aromatic vinyl compound include styrene compounds such as styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, o,p-dimethylstyrene, o-ethylstyrene, m-ethylstyrene and p-ethylstyrene. The aromatic vinyl units in the multi-component copolymer include styrene units, and preferably consist only of styrene units. The aromatic rings in the aromatic vinyl units, unless bonded to a neighboring unit, are not included on the copolymer main chain.

The aromatic vinyl compound may be of one type used alone, or two or more types may be used together. That is, the multi-component copolymer may contain one type of aromatic vinyl unit, or may contain two or more types. It is preferable for the content of aromatic vinyl units to be from 3 to 30 wt % of the overall multi-component copolymer. At an aromatic vinyl unit content of less than 3 wt % or more than 30 wt %, the lengths of the non-conjugated olefin moieties on the copolymer cannot be controlled, and so an improvement in durability due to non-conjugated olefin crystals is not achieved. The aromatic vinyl unit content is preferably from 3 to 30 wt %, more preferably from 5 to 25 wt %, and even more preferably from 10 to 20 wt %, of the overall multi-component copolymer.

The number of types of monomers in the multi-component copolymer is not particularly limited, so long as the multi-component copolymer includes conjugated diene units, non-conjugated olefin units and aromatic vinyl units. The multi-component copolymer may also include constituent units other than conjugated diene units, non-conjugated olefin units and aromatic vinyl units. The content of such other constituent units, from the standpoint of obtaining the desired effects, is preferably not more than 30 wt %, more preferably not more than 20 wt %, and even more preferably not more than 10 wt %, of the overall multi-component copolymer. The absence of any such other constituent units, that is, a content of 0 wt %, is especially preferred.

The multi-component copolymer is, at the very least, a multi-component copolymer having one type of conjugated diene unit, one type of non-conjugated olefin unit and one type of aromatic vinyl unit. Also, from the standpoint of imparting good failure characteristics, the multi-component copolymer is preferably a polymer obtained by polymerization using as the monomers at least one type of conjugated diene compound, one type of non-conjugated olefin compound and one type of aromatic vinyl compound.

The multi-component copolymer is more preferably a ternary copolymer consisting entirely of one type of conjugated diene unit, one type of non-conjugated olefin unit and one type of aromatic vinyl unit, and is even more preferably a ternary copolymer consisting entirely of 1,3-butadiene units, ethylene units and styrene units. Here, “one type of conjugated diene unit” encompasses conjugated diene units of differing bonding modes.

One major feature of the multi-component copolymer is that it contains conjugated diene units, non-conjugated olefin units and aromatic vinyl units and that the main chain consists entirely of acyclic structures. When the main chain has cyclic structures, the failure characteristics (especially the elongation at break) decrease. NMR spectroscopy may be used as the chief measurement means for ascertaining whether the main chain of the multi-component copolymer has cyclic structures. Specifically, when peaks attributable to cyclic structures present on the main chain (e.g., in the case of three-membered rings to five-member rings, peaks appearing at 10 to 24 ppm) are not observed, this indicates that the main chain of the multi-component copolymer consists solely of acyclic structures. The multi-component copolymer, as described below in the method of preparation therefor, can be synthesized in a single reactor, i.e., by one-pot synthesis, and thus can be prepared by a simplified process.

The multi-component copolymer has a polystyrene-equivalent weight-average molecular weight (Mw) of preferably from 10,000 to 10,000,000, more preferably from 100,000 to 9,000,000, and even more preferably from 150,000 to 8,000,000. By setting the Mw of the multi-component copolymer to at least 10,000, a standard strength for a golf ball material can be fully ensured; by setting Mw to 10,000,000 or less, a high workability can be maintained. The above weight-average molecular weight and molecular weight distribution are determined by gel permeation chromatography (GPC) using polystyrene as the reference material.

The chain structure of the multi-component copolymer is not particularly limited and may be suitably selected according to the intended purpose. For example, letting the conjugated diene units be A, the non-conjugated olefin units be B and the aromatic vinyl units be C, the copolymer may be a block copolymer having an Ax-By-Cz construction (wherein x, y and z are integers of 1 or more), a random copolymer with a construction in which A, B and C are randomly arranged, a tapered copolymer in which a random copolymer and a block copolymer are intermingled, or an alternating copolymer with an (A-B-C)w construction (wherein w is an integer of 1 or more). The multi-component copolymer may have a structure in which the conjugated diene units, non-conjugated olefin units and aromatic vinyl units are linearly connected (linear structure), or may have a structure in which at least the conjugated diene units, the non-conjugated olefin units or the aromatic vinyl units are connected so as to form a branched chain (branched structure). In cases where the multi-component copolymer has a branched structure, the branched chain may be made a binary or multi-component chain (i.e., the branched chain may include at least two from among the conjugated diene units, non-conjugated olefin units and aromatic vinyl units). Therefore, even among multi-component copolymers, a multi-component copolymer with a branched structure having a binary or multi-component branched chain can be clearly distinguished from a conventional graft copolymer in which the main chain and the side chains are each formed of a single and differing type of unit.

The multi-component copolymer production method, polymerization steps and production conditions used, such as the polymerization catalyst, may be as described in the specification of JP No. 6780827. It is preferable for the multi-component copolymer to be one polymerized by means of a gadolinium metallocene complex catalyst.

The outer cover layer has a material hardness on the Shore D hardness scale which is at least 35, preferably at least 40, and more preferably at least 42. The upper limit is 55 or less, preferably 52 or less, and more preferably 50 or less. When the material hardness of the outer cover layer falls outside of the above range, a sufficient spin rate-lowering effect on full shots with a driver (W #1) cannot be fully achieved and high controllability due to a high spin rate on approach shots is not obtained.

The outer cover layer has a thickness which, although not particularly limited, is preferably at least 0.4 mm, and more preferably at least 0.5 mm, and is preferably not more than 1.0 mm, and more preferably not more than 0.9 mm.

In this invention, the thickness relationship among the above-described intermediate layer, inner cover layer and outer cover layer preferably satisfies the following condition:

intermediate layer thickness>inner cover layer thickness<outer cover layer thickness.

Also, the ratio (a)/(b) between the inner cover layer thickness (a) and the outer cover layer thickness (b) is preferably at least 0.10, and more preferably at least 0.20, and is preferably not more than 0.60, and more preferably not more than 0.50. Outside of the above range, the spin rate on driver (W #1) shots may rise, as a result of which an increased distance may not be achieved, or the feel at impact may worsen.

The sum of the inner cover layer thickness (a) and the outer cover layer thickness (b), expressed as (a)+(b), is at least 0.6 mm, and preferably at least 0.7 mm. The upper limit value is not more than 1.2 mm, and preferably not more than 1.0 mm. When the sum of these thicknesses falls outside of the above range, a high spin rate cannot be obtained on approach shots with a club such as a sand wedge (SW), or the spin rate on driver (W #1) shots rises, resulting in a decreased distance.

The outer cover layer material has a rebound resilience, as measured according to JIS-K 6255, which is preferably at least 40%, and more preferably at least 45%. The upper limit value is preferably 60% or less, and more preferably 55% or less. When the rebound resilience is too low, the spin rate on driver shots may rise, resulting in a decreased distance, and the spin rate on approach shots may decrease. When the rebound resilience is too high, the high spin rate desired on approach shots may not be obtained.

A distinctive feature of the golf ball of this invention is that, letting (A) be the rebound resilience (%) of the inner cover layer material based on JIS-K 6255 and (B) be the rebound resilience (%) of the outer cover layer material based on JIS-K 6255, the ball satisfies the condition 0%<(A)−(B)<18%. By thus setting the rebound resilience of the inner cover layer so as to always be higher than the rebound resilience of the outer cover layer and also setting the difference between the rebound resiliences of these two layers within a specific range, the spin rate of the ball on approach shots is higher without any loss in the distance performance of the ball on driver (W #1) shots, in addition to which a good feel on impact is obtained, enabling the desired effects of the invention to be achieved. The difference between the rebound resiliences of the inner and outer cover layers, expressed as (A)−(B), is preferably at least 3%, more preferably at least 5%, and even more preferably at least 7%. The upper limit value is preferably not more than 17%. At a (A)−(B) value outside of the above range, the ball is not sufficiently receptive to spin on approach shots, and so the high spin rate desired cannot be obtained.

The outer cover layer is made of a resin material which is not particularly limited, although the use of a polyurethane resin material is preferred. Details on the polyurethane material are shown below.

The polyurethane has a structure which includes soft segments composed of a polymeric polyol (polymeric glycol) that is a long-chain polyol, and hard segments composed of a chain extender and a polyisocyanate. Here, the polymeric polyol serving as a starting material may be any that has hitherto been used in the art relating to polyurethane materials, and is not particularly limited. It is exemplified by polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. Specific examples of polyester polyols that may be used include adipate-type polyols such as polyethylene adipate glycol, polypropylene adipate glycol, polybutadiene adipate glycol and polyhexamethylene adipate glycol; and lactone-type polyols such as polycaprolactone polyol. Examples of polyether polyols include poly(ethylene glycol), poly(propylene glycol), poly(tetramethylene glycol) and poly(methyltetramethylene glycol). These polyols may be used singly or two or more may be used in combination.

It is preferable to use a polyether polyol as the polymeric polyol.

The long-chain polyol has a number-average molecular weight that is preferably in the range of 1,000 to 5,000. By using a long-chain polyol having a number-average molecular weight in this range, golf balls made with a polyurethane composition that have excellent properties, including a good rebound and good productivity, can be reliably obtained. The number-average molecular weight of the long-chain polyol is more preferably in the range of 1,500 to 4,000, and even more preferably in the range of 1,700 to 3,500.

Here and below, “number-average molecular weight” refers to the number-average molecular weight calculated based on the hydroxyl value measured in accordance with JIS-K1557.

The chain extender is not particularly limited; any chain extender that has hitherto been employed in the art relating to polyurethanes may be suitably used. In this invention, low-molecular-weight compounds with a molecular weight of 2,000 or less and having on the molecule two or more active hydrogen atoms capable of reacting with isocyanate groups may be used. Of these, preferred use can be made of aliphatic diols having from 2 to 12 carbon atoms. Specific examples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. The use of 1,4-butylene glycol is especially preferred.

Any polyisocyanate hitherto employed in the art relating to polyurethanes may be suitably used without particular limitation as the polyisocyanate. For example, use can be made of one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate, 1,4-bis(isocyanatomethyl)cyclohexane and dimer acid diisocyanate. However, depending on the type of isocyanate, crosslinking reactions during injection molding may be difficult to control.

The ratio of active hydrogen atoms to isocyanate groups in the polyurethane-forming reaction may be suitably adjusted within a preferred range. Specifically, in preparing a polyurethane by reacting the above long-chain polyol, polyisocyanate and chain extender, it is preferable to use the respective components in proportions such that the amount of isocyanate groups included in the polyisocyanate per mole of active hydrogen atoms on the long-chain polyol and the chain extender is from 0.95 to 1.05 moles.

The method of preparing the polyurethane is not particularly limited. Preparation using the long-chain polyol, chain extender and polyisocyanate may be carried out by either a prepolymer process or a one-shot process via a known urethane-forming reaction. Of these, melt polymerization in the substantial absence of solvent is preferred. Production by continuous melt polymerization using a multiple screw extruder is especially preferred.

It is preferable to use a thermoplastic polyurethane material as the polyurethane, with an ether-based thermoplastic polyurethane material being especially preferred. The thermoplastic polyurethane material used may be a commercial product, illustrative examples of which include those available under the registered trademark Pandex from DIC Covestro Polymer, Ltd., and those available under the trade name Resamine from Dainichiseika Color & Chemicals Mfg. Co., Ltd.

Various optional additives such as pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers and mold lubricants may be included in the above-described resin materials for the intermediate layer, inner cover layer and outer cover layer. Each of the above resin materials may be obtained by mixing together the ingredients using various types of mixers, such as a kneading-type single-screw or twin-screw extruder, a Banbury mixer or a kneader.

The production of a sphere (unpainted golf ball) that includes the above-described core, intermediate layer, inner cover layer and outer cover layer in the invention may be carried out in the usual manner, such as by a known injection molding process or heat compression molding process. For example, a golf ball having a multilayer construction can be produced by successively injection-molding the intermediate layer material and the inner cover layer material over the core in respective injection molds so as to obtain first an intermediate layer-encased sphere and then an inner cover layer-encased sphere, and then injection-molding the material for the outer cover layer serving as the outermost layer over the inner cover layer-encased sphere. Alternatively, production of the golf ball may involve forming the respective encasing layers by enclosing the sphere to be encased within two pre-molded hemispherical half-cups and then molding under applied heat and pressure.

From the standpoint of the aerodynamic performance of the ball, numerous dimples are provided on the surface of the outermost layer. Although the number of dimples formed on the surface of the outermost layer is not particularly limited, to enhance the aerodynamic performance and increase the distance traveled by the ball, the number of dimples is preferably at least 250, more preferably at least 270, even more preferably at least 290, and most preferably at least 300. The number of dimples is preferably not more than 400, more preferably not more than 380, and even more preferably not more than 360.

In this invention, a coating layer is formed on the cover surface. A two-part curable urethane coating may be suitably used as the coating material to form this coating layer. Illustrative examples of such two-part curable urethane coatings include ones which include a base resin composed primarily of polyol resin and a curing agent composed primarily of polyisocyanate.

The method used to apply the above coating onto the cover surface and form the coating layer is not particularly limited. A known method may be used for this purpose. For example, use can be made of a suitable method such as spray painting with an air gun or electrostatic painting.

The thickness of the coating layer is not particularly limited, although it is typically from 8 to 22 μm, and preferably from 10 to 20 μm.

The golf ball of the invention can be made to conform to the Rules of Golf for play. The inventive ball may be formed to a diameter which is such that the ball does not pass through a ring having an inner diameter of 42.672 mm but is not more than 42.80 mm, and to a weight which is preferably between 45.0 and 45.93 g.

EXAMPLES

The following Examples and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.

Examples 1 to 5, Comparative Examples 1 to 11

Cores having a diameter of 38.7 mm are produced by preparing a core rubber composition common to all of the examples using the formulation shown in Table 1, and molding and vulcanizing the composition.

TABLE 1 Rubber formulation pbw cis-1,4-Polybutadiene 100 Zinc acrylate 27 Zinc oxide 4.0 Barium sulfate 16.5 Antioxidant 0.2 Organic peroxide (1) 0.6 Organic peroxide (2) 1.2 Zinc salt of 0.3 pentachlorothiophenol Zinc stearate 1.0

Details on the above core material are given below.

-   -   cis-1,4-Polybutadiene: Available under the trade name “BR 01”         from JSR Corporation     -   Zinc acrylate: Available from Nippon Shokubai Co., Ltd.     -   Zinc oxide: Available from Sakai Chemical Co., Ltd.     -   Barium sulfate: Available from Sakai Chemical Co., Ltd.     -   Antioxidant: Available under the trade name “Nocrac NS-6” from         Ouchi Shinko Chemical Industry Co., Ltd.     -   Organic Peroxide (1): Dicumyl peroxide, available under the         trade name “Percumyl D” from NOF Corporation     -   Organic Peroxide (2): Mixture of         1,1-di(tert-butylperoxy)cyclohexane and silica, available under         the trade name “Perhexa C-40” from NOF Corporation     -   Zinc stearate: Available from NOF Corporation

Intermediate Layer

The resin material for the intermediate layer is injection-molded over the 38.7 mm diameter core, thereby producing an intermediate layer-encased sphere having an intermediate layer thickness of 1.0 mm, 1.2 mm or 1.4 mm. The resin material for this intermediate layer is a blend having a resin formulation common to all of the examples which is composed of 50 parts by weight of a sodium-neutralized ethylene-unsaturated carboxylic acid copolymer having an acid content of 18 wt % and 50 parts by weight of a zinc-neutralized ethylene-unsaturated carboxylic acid copolymer having an acid content of 15 wt %, for a total of 100 parts by weight.

Inner Cover Layer

Next, using resin materials Nos. 1 to 8 shown in Table 2 (Examples 1 to 5 and Comparative Examples 1 to 3) and Table 3 (Comparative Examples 4 to 11), an inner cover layer is formed over the intermediate layer-encased sphere in each example. The method of formation consists of pre-molding a pair of hemispherical half-cups from the inner cover layer material, enclosing the intermediate layer-encased sphere within the half-cups and molding under applied heat and pressure.

Details on the ingredients included in the compositions shown in Tables 2 and 3 are given below.

-   -   No. 1: see description below of Multi-Component Copolymer A     -   No. 2: see description below of Multi-Component Copolymer B     -   No. 3: a thermoplastic polyether ester elastomer available as         Hytrel 3001 from DuPont-Toray Co., Ltd.     -   No. 4: a thermoplastic polyether ester elastomer available as         Hytrel 2401 from DuPont-Toray Co., Ltd.     -   No. 5: HPF 2000, from The Dow Chemical Company     -   No. 6: Nucrel AN 4319, available from Dow-Mitsui Polychemicals         Co., Ltd.     -   No. 7: Nucrel AN 42012, available from Dow-Mitsui Polychemicals         Co., Ltd.     -   No. 8: S.O.E. 51611, a styrene-ethylene/butylene-styrene block         copolymer (SEBS) available from Asahi Kasei Corporation

Multi-Component Copolymer A

Eighty grams of styrene and 600 mL of toluene are added to a thoroughly dried 1,000 mL stainless steel pressure reactor.

A catalyst solution is prepared by charging a glass vessel within a glovebox under a nitrogen atmosphere with 0.25 mmol of a mono(bis(1,3-tert-butyldimethylsilyl)indenyl) bis(bis(dimethylsilyl)amide gadolinium complex (1,3-[(t-Bu)Me₂Si]₂C₉H₅Gd[N(SiHMe₂)₂]₂), 0.275 mmol of dimethylanilinium tetrakis(pentafluorophenyl) borate [Me₂NHPhB(C₆F₅)₄] and 1.1 mmol of diisobutylaluminum hydride, and adding 40 mL of toluene. The catalyst solution is added to the stainless steel pressure reactor and the temperature is raised to 70° C.

Next, ethylene is introduced under a pressure of 1.5 MPa to the stainless steel pressure reactor, in addition to which 80 mL of a toluene solution containing 20 g of 1,3-butadiene is introduced to the stainless steel pressure reactor over a period of 8 hours, and copolymerization is carried out at 70° C. for a total of 8.5 hours.

The reaction is stopped by adding 1 mL of a 5 wt % isopropanol solution of 2,2′-methylene-bis(4-ethyl-6-t-butylphenol) (NS-5) to the stainless steel pressure reactor.

The copolymer is then separated off using a large amount of methanol, and vacuum dried at 50° C. to give Copolymer A.

Multi-Component Copolymer B

Sixty-five grams of styrene and 600 mL of toluene are added to a thoroughly dried 1,000 mL stainless steel pressure reactor.

A catalyst solution is prepared by charging a glass vessel within a glovebox under a nitrogen atmosphere with 0.26 mmol of a mono(bis(1,3-tert-butyldimethylsilyl)indenyl) bis(bis(dimethylsilyl)amide gadolinium complex (1,3-[(t-Bu)Me₂Si]₂C₉H₅Gd[N(SiHMe₂)₂]₂), 0.283 mmol of dimethylanilinium tetrakis(pentafluorophenyl) borate [Me₂NHPhB(C₆F₅)₄] and 0.83 mmol of diisobutylaluminum hydride, and adding 40 mL of toluene. The catalyst solution is added to the stainless steel pressure reactor and the temperature is raised to 70° C.

Next, ethylene is introduced under a pressure of 1.5 MPa to the stainless steel pressure reactor, in addition to which 120 mL of a toluene solution containing 24 g of 1,3-butadiene is introduced to the stainless steel pressure reactor over a period of 8 hours, and copolymerization is carried out at 70° C. for a total of 8.5 hours.

The reaction is stopped by adding 1 mL of a 5 wt % isopropanol solution of 2,2′-methylene-bis(4-ethyl-6-t-butylphenol) (NS-5) to the stainless steel pressure reactor.

The copolymer is then separated off using a large amount of methanol and vacuum dried at 50° C. to give Copolymer B.

The butadiene, ethylene and styrene contents (wt %) of each of the copolymers, i.e., Multi-Component Copolymer A and Multi-Component Copolymer B, are measured and evaluated by the following methods.

Butadiene, Ethylene and Styrene Contents

The butadiene, ethylene and styrene contents of each copolymer are determined by ¹H-NMR spectroscopy. The results are as follows.

-   -   Multi-Component Copolymer A: butadiene/ethylene/styrene=14/70/16         (wt %); weight-average molecular weight (Mw), 273×10³     -   Multi-Component Copolymer B: butadiene/ethylene/styrene=16/72/12         (wt %); weight-average molecular weight (Mw), 338×10³

Outer Cover Layer

Next, a urethane resin material common to all of the examples (an ether-type thermoplastic polyurethane available from DIC Covestro Polymer, Ltd. under the registered trademark Pandex; Shore D hardness, 47) is injection-molded over the sphere obtained by encasing the intermediate layer-encased sphere with the inner cover layer, thereby producing golf balls having an outer cover layer of a given thickness such as to give, in all of the examples, a ball diameter of 42.7 mm. At this time, although not particularly shown in the diagram, dimples are formed on the cover surface in an arrangement common to each Example and Comparative Example. Also, in each Example and Comparative Example, a two-part curable polyurethane resin composition common to all the examples is applied to a thickness of 15 μm on the cover surface.

The material hardnesses and rebound resiliences of the above golf balls are measured as described below, giving the values shown in Table 2 (Examples 1 to 5 and Comparative Examples 1 to 3) and Table 3 (Comparative Examples 4 to 11).

Material Hardnesses (Shore D Hardnesses) of Inner Cover Layer and Outer Cover Layer

The resin material for each cover layer is formed into 2 mm-thick sheets and left to stand for at least two weeks, following which the Shore D hardness is measured in accordance with ASTM D2240-95.

Rebound Resilience

The resin material is formed into 2 mm-thick sheets with a press, stacked to a thickness of 4 mm and temperature-conditioned at 23±1° C., following which measurement is carried out according to JIS-K 6255 (2013) using a tripsometer (the rebound resilience is measured after adjusting the angle of impact in JIS-K 6255 to 40 degrees).

The spin performance on driver (W #1) shots and approach shots and the feel on approach shots for each golf ball are evaluated by the following methods. The results are presented in Table 2 (Examples 1 to 5 and Comparative Examples 1 to 3) and Table 3 (Comparative Examples 4 to 11).

Spin Performance on Driver Shots

A driver (W #1) is mounted on a golf swing robot and the backspin rate of the ball immediately after being struck at a head speed (HS) of 45 m/s is measured with a launch monitor and rated according to the criteria shown below. The driver used is the TourB XD-3 (loft angle, 9.5°) manufactured by Bridgestone Sports Co., Ltd.

Rating Criteria:

-   -   Exc: Spin rate is less than 3,000 rpm     -   Good: Spin rate is at least 3,000 rpm but less than 3,100 rpm     -   Fair: Spin rate is at least 3,100 rpm but less than 3,200 rpm     -   NG: Spin rate is 3,200 rpm or more

Spin Rate on Approach Shots

A sand wedge (SW) is mounted on a golf swing robot and the backspin rate of the ball immediately after being struck at a head speed (HS) of 10 m/s is measured with a launch monitor and rated according to the criteria shown below. The sand wedge used is the TourB XW-B (loft angle, 56°) manufactured by Bridgestone Sports Co., Ltd.

Rating Criteria:

-   -   Exc: Spin rate is 3,200 rpm or more     -   Good: Spin rate is at least 3,150 rpm but less than 3,200 rpm     -   Fair: Spin rate is at least 3,050 rpm but less than 3,150 rpm     -   NG: Spin rate is less than 3,050 rpm

Evaluation of Feel

A sensory evaluation of the feel of the ball when struck on approach shots is carried out. The club used is the sand wedge TOUR B XW-B (loft angle, 56°) manufactured by Bridgestone Sports Co., Ltd. Evaluation is carried out according to the following criteria when the ball is struck by an amateur golfer.

Rating Criteria:

-   -   Exc: Very good feel     -   Good: Good feel     -   NG: Poor feel

The physical shock (hardness) and sound perceived by the golfer when the ball comes into contact with the clubface, and the manner in which the ball comes off the clubface, are included in the rating of the ball feel on impact.

TABLE 2 Comparative Example Example 1 2 3 4 5 1 2 3 Core Composition Rubber composition common to all examples Diameter (mm) 38.7 38.7 38.7 38.7 38.7 38.7 38.7 38.7 Intermediate Composition Ionomer resin common to all examples layer Shore D hardness 66 66 66 66 66 66 66 66 Diameter (mm) 40.7 41.1 40.7 41.1 41.3 40.7 41.1 41.3 Thickness (mm) 1.0 1.2 1.0 1.2 1.3 1.0 1.2 1.3 Inner cover Composition No. 1 No. 1 No. 2 No. 2 No. 2 — — — layer Shore D hardness 30 30 35 35 35 — — — Rebound resilience (%) (A) 66.9 66.9 57.1 57.1 57.1 — — — Thickness (mm) 0.2 0.2 0.2 0.2 0.2 — — — Sphere I Diameter (mm) 41.1 41.5 41.1 41.5 41.7 — — — Outer cover Composition Urethane resin common to all examples layer Shore D hardness 47 47 47 47 47 47 47 47 Rebound resilience (%) (B) 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 Ball Diameter 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Thickness (mm) 0.8 0.6 0.8 0.6 0.5 1.0 0.8 0.7 Combined thickness of 1.0 0.8 1.0 0.8 0.7 — — — inner and outer cover layers (mm) Inner cover layer thickness/ 0.25 0.33 0.25 0.33 0.40 — — — Outer cover layer thickness Rebound resilience difference (A) − (B) 16.9 16.9 7.1 7.1 7.1 — — — Ball Spin rate on driver shots (rpm) 3,034 2,980 3,008 2,954 2,921 2,953 2,865 2,833 evaluation Rating Good Exc Good Exc Exc Exc Exc Exc Spin rate on approach shots (rpm) 3,229 3,179 3,276 3,225 3,196 3,137 3,068 3,022 Rating Exc Good Exc Exc Good Fair Fair NG Feel on approach shots Good Exc Good Good Exc NG Good Exc Sphere I: Sphere obtained by encasing core with intermediate layer and inner cover layer

TABLE 3 Comparative Example 4 5 6 7 8 9 10 11 Core Composition Rubber composition common to all examples Diameter (mm) 38.7 38.7 38.7 38.7 38.7 38.7 38.7 38.7 Intermediate Composition Ionomer resin common to all examples layer Shore D hardness 66 66 66 66 66 66 66 66 Diameter (mm) 40.7 41.1 40.7 41.1 40.7 40.7 40.7 40.7 Thickness (mm) 1.0 1.2 1.0 1.2 1.0 1.0 1.0 1.0 Inner cover Composition No. 3 No. 3 No. 4 No. 4 No. 5 No. 6 No. 7 No. 8 layer Shore D hardness 31 31 42 42 50 30 52 24 Rebound resilience (%) (A) 69.4 69.4 64.4 64.4 71.8 49.3 25.1 22.2 Thickness (mm) 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Sphere I Diameter (mm) 41.1 41.5 41.1 41.5 41.1 41.1 41.1 41.1 Outer cover Composition Urethane resin common to all examples layer Shore D hardness 47 47 47 47 47 47 47 47 Rebound resilience (%) (B) 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 Ball Diameter 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Thickness (mm) 0.8 0.6 0.8 0.6 0.8 0.8 0.8 0.8 Combined thickness of 1.0 0.8 1.0 0.8 1.0 1.0 1.0 1.0 inner and outer cover layers (mm) Inner cover layer thickness/ 0.25 0.33 0.25 0.33 0.25 0.25 0.25 0.25 Outer cover layer thickness Rebound resilience difference (A) − (B) 19.4 19.4 14.4 14.4 21.8 −0.7 −24.9 −27.8 Ball Spin rate on driver shots (rpm) 2,860 2,809 3,071 3,016 3,285 3,106 2,909 3,052 evaluation Rating Exc Exc Good Good NG Fair Exc Good Spin rate on approach shots (rpm) 3,033 2,973 3,195 3,132 3,213 3,061 3,021 3,136 Rating NG NG Good Fair Exc Fair NG Fair Feel on approach shots Good Exc NG Good NG Good NG Good

As demonstrated by the results in Tables 2 and 3, the golf balls of Comparative Examples 1 to 11 are inferior in the following respects to the golf balls according to the present invention that are obtained in Examples 1 to 5.

In Comparative Example 1, an inner cover layer is not formed. As a result, the feel of the ball is poor and the spin rate on approach shots is somewhat low.

In Comparative Example 2, an inner cover layer is not formed. As a result, the spin rate on approach shots is somewhat low.

In Comparative Example 3, an inner cover layer is not formed. As a result, the spin rate on approach shots is low.

In Comparative Example 4, the difference in rebound resilience between the inner cover layer and the outer cover layer exceeds 18%. As a result, the spin rate on approach shots is low.

In Comparative Example 5, the difference in rebound resilience between the inner cover layer and the outer cover layer exceeds 18%. As a result, the spin rate on approach shots is low.

In Comparative Example 6, the inner cover layer has a material hardness on the Shore D hardness scale that exceeds 40. As a result, the feel of the ball on approach shots is poor.

In Comparative Example 7, the inner cover layer has a material hardness on the Shore D hardness scale that exceeds 40. As a result, the spin rate on approach shots is somewhat low.

In Comparative Example 8, the inner cover layer has a material hardness on the Shore D hardness scale which exceeds 40 and the difference in rebound resilience between the inner cover layer and the outer cover layer exceeds 18%. As a result, the spin rate on shots with a driver (W #1) is somewhat high and the feel is poor.

In Comparative Example 9, the rebound resilience of the outer cover layer is higher than the rebound resilience of the inner cover layer. As a result, the spin rate on shots with a driver (W #1) is somewhat high and the spin rate on approach shots is somewhat low.

In Comparative Example 10, the inner cover layer has a material hardness on the Shore D hardness scale which exceeds 40 and the rebound resilience of the outer cover layer is higher than the rebound resilience of the inner cover layer. As a result the spin rate on approach shots is low and the feel is poor.

In Comparative Example 11, the rebound resilience of the outer cover layer is higher than the rebound resilience of the inner cover layer. As a result, the spin rate on approach shots is somewhat low.

Japanese Patent Application No. 2022-062754 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A golf ball comprising a core of one or more layer, an intermediate layer and a cover, wherein the cover consists of an inner cover layer and an outer cover layer, the inner cover layer being made of a material having a hardness on the Shore D hardness scale of from 20 to 40, the outer cover layer being made of a material having a hardness on the Shore D hardness scale of from 35 to 55 and the inner and outer cover layers having a combined thickness of from 0.6 to 1.2 mm, and wherein, letting (A) be the rebound resilience (%) of the inner cover layer material based on JIS-K 6255 and (B) be the rebound resilience (%) of the outer cover layer material based on JIS-K 6255, the ball satisfies the condition 0%<(A)−(B)<18%.
 2. The golf ball of claim 1, wherein the outer cover layer material comprises a resin component which is a thermoplastic polyurethane elastomer.
 3. The golf ball of claim 1, wherein the inner cover layer material comprises a resin component which is a thermoplastic elastomer.
 4. The golf ball of claim 1, wherein the inner cover layer material comprises a resin component which is a multi-component copolymer having conjugated diene units, non-conjugated olefin units and aromatic vinyl units, the conjugated diene units including butadiene units, the non-conjugated olefin units including units selected from ethylene units, propylene units and 1-butene units, the aromatic vinyl units including styrene units, and the content of the conjugated diene units in the multi-component copolymer being at least 5 wt %.
 5. The golf ball of claim 4, wherein the content of the conjugated diene units in the multi-component copolymer is at least 10 wt %.
 6. The golf ball of claim 4, wherein the content of the non-conjugated olefin units in the multi-component copolymer is up to 90 wt %.
 7. The golf ball of claim 4, wherein the content of the aromatic vinyl units in the multi-component copolymer is up to 30 wt %.
 8. The golf ball of claim 4, wherein the non-conjugated olefin units are ethylene units.
 9. The golf ball of claim 4, wherein the multi-component copolymer is a copolymer polymerized with a gadolinium metallocene complex catalyst.
 10. The golf ball of claim 1, wherein the inner cover layer has a thickness of from 0.1 to 0.5 mm.
 11. The golf ball of claim 1, wherein the intermediate layer, the inner cover layer and the outer cover layer have a thickness relationship therebetween which satisfies the following condition: intermediate layer thickness>inner cover layer thickness<outer cover layer thickness.
 12. The golf ball of claim 1, wherein the ratio (a)/(b) between the inner cover layer thickness (a) and the outer cover layer thickness (b) is from 0.10 to 0.60. 